A compact and thin-type image pickup apparatus has come to be mounted on a mobile terminal representing a compact and thin-type electronic hardware such as a cell phone and PDA (Personal Digital Assistant), whereby, it has become possible to transmit mutually not only voice information but also image information to a remote location.
As image pickup elements used for these image pickup apparatuses, a solid-state imaging device such as an image sensor of a CCD type and an image sensor of a CMOS type are used. In recent years, increase of pixel numbers of the image pickup element have been advanced, and enhancement of its resolution and performance have been attained. As a lens for forming an image of a photographic object on the imaging device, a lens made by resin that is suitable for mass production has come to be used for further cost reduction. In addition, the lens made by resin is excellent in terms  of its workability and has satisfied requirements for enhancement of its performance, by being formed in aspheric shape because its workability is excellent.
As an image pickup lens of this kind used for an image pickup apparatus housed in a mobile terminal, an optical system of a three-element structure including three plastic lenses and an optical system of a three-element structure including one glass lens and two plastic lenses are widely known. However, a demand for further compactness for these image pickup lenses and a demand for mass productivity required for the mobile terminal are becoming stronger, and the compatibility between both demands is becoming more difficult.
To solve the problem, there has been proposed a method to produce a large number of lens modules as followings. There is provided a glass substrate in a size of several inches, which is formed in a parallel flat plate. A large amount of lens elements are simultaneously formed on the glass substrate through a replica method. Then, the glass substrate (lens wafer) on which a large number of lens elements are formed is combined with a sensor wafer and is cut off to produce a large number of lens modules. Lenses manufactured by this method are called wafer-scale lenses, and lens modules manufactured by this method are called wafer-scale lens modules.
Additionally to the method to produce a large number of lens modules, there has recently been suggested a method to mount the large number of lens modules on a substrate at low cost, as followings. Lens modules are arranged together with IC (Integrated Circuit) chips and other electronic parts on a substrate on which a solder is potted in advance. By adding reflow processing (heating processing) to the substrate to melt the solder as the lens modules are arranged thereon, the electronic components and the lens modules are simultaneously mounted on the substrate. Image pickup lenses that withstand reflow processing and are excellent in heat resistance are also demanded.
As the image pickup lens of this kind, there are proposed lenses shown in JP-B Nos. 3929479 and 3976781 which disclose lens blocks of a two-element structure. However, in these lenses, aberration correcting power is insufficient, and it is difficult to say that these image pickup lenses sufficiently copes with a solid-state imaging device with lager number of pixels. In particular, chromatic aberration is hardly corrected in these lenses, thereby, an image pickup lens shown in JP-A No. 2006-323365 in which a diffractive surface is applied on a lens substrate, and an image pickup lens in JP-A No. 2008-233884 in which an object-side lens and an image-side lens in a lens block are formed of different materials are also proposed.
However, in the image pickup lens shown in JP-A No. 2006-323365, employing the diffraction surface rises the degree of difficulty in manufacturing, and decreases diffraction efficiency  for a wavelength other than a design wavelength. It generates a diffracted light of unwanted order, resulting in a problem of ghost. On the other hand, a structure of the image pickup lens in JP-A No. 2008-233884 enables to correct various aberrations in an excellent condition. However, the power of the first lens block of the image pickup lens is large in order to shorten the total length of the image pickup lens, and it easily enlarges sensitivity to a back focus when the first lens block has a thickness error. To solve the problem, controlling the dimension of the first lens block can be considered. However, lens portions formed on the opposing surfaces of a lens substrate in the first lens block are formed of resins with different refractive indexes, and it requires to control thicknesses of the lens portions on the opposing surfaces of the lens substrate and a thickness of the lens substrate from a viewpoint of manufacturing, which can be a primary factor to worsen a mass-productivity of the image pickup lens.